X-ray imaging and, in particular, computer tomography systems enable spectral imaging via traditional energy-integrating X-ray detectors, by examining an object under examination, as a rule a patient, using two divergent X-ray spectra. This can, for example, be performed via two X-ray sources operating in parallel or via fast kV switching at only one X-ray source. Typically, an 80-kV spectrum and a 140-kV spectrum can be used.
In addition, the spectral separation of the energy spectra can be reinforced by way of corresponding pre-filtering, e.g. with dual-energy imaging with the X-ray source which is operated with a higher tube voltage of, for example, 140 kV, a tin filter is regularly used to clean the energy spectrum of the X-ray source of low-energy photons. Inherent in this method is the temporal and/or spatial separation of measurements, with the consequence that known algorithms for material breakdown can only operate in the image space.
In contrast, spectrally sensitive X-ray detectors permit the simultaneous recording of X-ray attenuation data in one and the same projection direction with regard to an object under examination in two or more different spectral regions. In principle, this enables flexible further processing of the X-ray attenuation data and can also advantageously reduce the dose to which the patient is exposed. However, the spectral detector response function of an actual, spectrally sensitive, photon-counting X-ray detector is not perfect as it is impaired by physical and electronic effects. These include, inter alia, in particular the effects of pulse pileup, charge sharing, K-escape, Compton scattering and charge trapping.
With pulse pileup, photons striking the detector are simultaneously or quasi-coincidently converted into overlapping pulses which are detected as a resulting pulse of correspondingly higher energy and result in the loss of a counting signal. With charge sharing, a charge cloud generated by an incident photon is at least partially transferred to one or more adjacent pixels in the detector material. As a result, counting signals in different pixels are detected by a photon in energies below the quantum energy of the incident photon.
If a hole generated by an incident photon for each photoelectric effect on an inner shell of an atom of the detector material is filled up by an electron of a higher shell, characteristic radiation in the form of (fluorescent) X-rays of the corresponding K-junction is released. At best, this radiation causes a pulse pileup in the same pixel. But due to its stochastic directional distribution, it may also be detected in another pixel or not at all. In both the aforementioned cases, the counting signals are distorted.
The Compton effect, in which the photons undergoing a change of direction only deposit a portion of their energy in the detector material, results in a false detection site, insufficient detected energy and/or no detection of this photon at all. As a result of impurities or lattice defects in the detector material, charges generated with charge trapping may be trapped and only detected with reduced energy with a time lag. In short, the counting signals emitted by the counting X-ray detector in the individual energy bins or energy windows only partially correspond to the actual incident X-rays on the detector in these energy fields. This results in the spectral resolution of adjacent spectral regions of the incident energy spectrum being impaired so that they can only be mapped insufficiently or not at all. As a result, the basic “high-energy capacity” and in particular the “dual-energy capacity” of a spectrally sensitive X-ray detector is limited.
In addition, there are currently many applications and/or examinations by way of X-ray imaging in the ultra-low dosage range. These are applications which are or must be performed with a lower X-ray dose applied to the object under examination than would be obtainable by the minimum X-ray source current. The main application fields are lung cancer screening or pediatrics.
For an energy-integrating detector, filters are used for this purpose which reduce the X-ray intensity to such an extent that an examination is nonetheless possible in the ultra-low dosage range, for example, a tin filter is used at a tube voltage of 100 KV. There are currently no optimized filters for examinations in the ultra-low dosage range with spectrally sensitive detectors.